1. Field of the Invention
This invention relates to fuel injection pump control systems in diesel engines, and more particularly to a fuel injection pump control system in a diesel engine, suitable for electronically controlling fuel injection time.
2. Description of the Prior Art
Heretofore, in a known fuel injection system in which fuel injection time is electronically controlled, fuel injection time is controlled by an actuator displaced by the pressure of fuel from a feed pump and the fuel pressure is regulated by means of a solenoid valve.
In the arrangement of the system of the type described, the actuator is further displaced by the pressure received from a fuel pump, and the displacement of the actuator is not determined solely in response to a control signal applied to the solenoid valve. Thus, the dispersion of the fuel injection time varies in accordance with the variation in fuel pressure and the like. To obviate these problems, there is provided a sensor for measuring the displacement of the actuator, and a signal emitted from this sensor is fed back to an electronic circuit, whereby the displacement of the actuator controls the fuel injection time.
FIG. 1 is a sectional view showing the conventional fuel injection time regulating mechanism in a fuel injection pump. The fuel injection pump includes a fuel flow rate regulating mechanism by a spill position adjustment in addition to the injection time regulating mechanism shown in the drawing, however, this portion does not relate to the present invention, so that illustration and description thereof will be omitted.
A cam shaft 22 is connected to a drive shaft 20 through a coupling 21. When the cam shaft 22 is driven by the drive shaft 20, a cam 23 rotates. The cam 23 is provided thereon with corners, and when one of these corners abuts against a roller 24, the cam shaft 22 is pushed to the right in the drawing. The right end portions of the cam shaft 22 has a plunger 25 of an injection pump, to which is fed fuel under high pressure. The timing at which the fuel fed to the plunger 25 is blown out to the outside can be determined by changing the abutted position of one of the corners of the cam 23 against the roller 24. Description will be given of the manner of determining this injection timing with reference to FIG. 2.
FIG. 2 is a sectional view of FIG. 1 showing the conventional timing mechanism. The roller 24 is affixed to a portion of a ring 27 through a roller shaft 26, and the ring 27 is supported by a portion of the main body of the pump in a condition where the ring 27 is rotatable through an angle of small degrees to the right and left from the axial line of the drive shaft 20. The forward end portion of the roller shaft 26 is rotatably engaged with a timer cylinder 29 and a plunger 30 through a ball 28. The plunger 30 is determined in its position by a balance of power in a timer pressure chamber 31. In addition, a rod 32 is connected to an end portion of the plunger 30 opposite to the pressure chamber 31, and further, to a core 34 combined with a displacement meter coil 33 through rod 32 to constitute a variable inductance type displacement meter. The rod 32 is coupled thereonto with a spring 35 for rendering a biasing force against the pressure from the pressure chamber 31. In addition, a discharge port 36 is provided for rendering the waste pressure from the plunger 30 to the intake side of the pump 37, and a port 38 is provided for taking fuel into the timer cylinder 31.
FIG. 3 shows the fuel pressure system and the electric signal system under the conventional fuel injection timing control. The fuel pressure-fed from the pump 37 is fed to a high pressure pump through a discharge piping 50, and the surplus fuel in the high pressure pump is returned to an intake piping 51. Furthermore, part of the fuel in the discharge piping 50 is diverted to branch piping 53 and applied to pressure chamber 31 of the timer through a stationary throttle 54. Part of the fuel, which has passed through the fixed throttle 54 is returned to the intake piping 51 through a solenoid valve 55. The pressure in the pressure chamber 31 of the timer is produced by the stationary throttle 54 and the solenoid valve 55, and a fuel injection time is determined by a position where the pressure is balanced with the spring 35.
The solenoid valve 55 is driven by an amplifier 56, and an output signal from a comparator 58, comparing a fuel time command signal Sj with an output from a measurement signal converter 57, is used as a control command to the amplifier 56. The measurement signal converter 57 is used for wave form converting the output signal from the variable inductance type displacement meter 33.
The wave form of the signal emitted from the amplifier 56 is a rectangular wave having a cycle F and an ON-time P as shown in FIG. 4. The average value of the fuel flow rate passing through the solenoid valve 55 is determined by a duty ratio P/F of the rectangular wave. The cycle F is normally selected to be a constant value.
FIG. 5 is a characteristic diagram showing the relationship between the duty ratio P/F and the fuel injection time, in which the ON-time P is varied so as to change the duty ratio, so that the fuel injection time can be controlled. However, as apparent from the drawing, the fuel injection time is greatly influenced by the feed pressure of the pump 37. In order to minimize this change, the displacement meter 33 is provided, and an output therefrom is made to be a feedback signal.
As described above, heretofore, there have been disadvantages in that the actuator is further displaced by the pressure received from the fuel pump, and the displacement of the actuator is not determined solely by the control signal applied to the solenoid valve. Thus, the dispersion of the fuel injection time varies in accordance with the variation in fuel pressure. In order to compensate for this disadvantage, a sensor for measuring the displacement of the actuator and a signal processing circuit are provided to constitute a feedback system, which, however, has presented the disadvantage that construction becomes complicated, the cost of product is increased and the reliability is lowered.